1. Field
This invention relates to stereoscopic imaging systems. More specifically, this invention discloses an improved stereoscopic television system which has a great degree of compatability with the existing commercial television infrastructure. Compared with prior art systems using sequential display of right and left picture information, it eliminates flicker and spurious temporal parallax effects, while having greatly increased brightness.
In the following description of the prior art, I describe and identify an effect which I have discovered, known as "spurious temporal parallax," which has led me to the disclosure of this invention. Specifically, where left and right images are taken out of time phase or spaced a predetermined distance apart (such as 0.008 seconds), a temporal dislocation of the spatial orientation of objects can occur. Such spurious effects arise because objects will move some finite distance in the time interval between the right and left shutter or scanning operation. This will produce unwanted motion and consequent depth reception in the respective image field. The brain in turn interprets this as stereoscopic information since it is turned into parallax information. The reader will understand that insofar as novelty herein is set forth, I specifically claim as a part of my invention the discovery of this effect as it relates to the problem of stereoscopic television transmission and reception. The identification of this problem as it relates to stereoscopic television leads to my solution. It goes without saying that the identification and proper statement of the problem to be solved oft times is the equivalent of invention.
2. State of the Art
Various means have been proposed for stereoscopic television systems. Whatever their virtues the technology is most deficient in image display. A number of inventions, such as U.S. Pat. Nos. 3,184,630 (Geer), 3,674,921 (Goldsmith) and 4,214,257 (Yamauchi) have suggested autostereoscopic means using interdigitized stereopairs. By juxtaposing lenticular elements in front of the CRT phosphur screen, with appropriately placed vertical image stripes of interdigitized stereopairs, it is hoped that successful selection of the left and right images may take place at the surface of the screen. However, as is well known in the art, this type of parallax stereogram display results in relatively narrow viewing zones, plus unwanted pseudoscopic zones, which require careful placement of the eyes in a particular stereoscopic zone. This results in restrictions in the number of viewers and on viewer head movement which may possibly find uses in certain limited applications, but such systems are very imperfect manifestations of the art.
One possible approach illustrated by U.S. Pat. No. 4,062,045 (Iwane), is to increase the number of image channels in an attempt to broaden the stereoscopic viewing zones. But such a technique would entail increasing the transmission bandwidth and lead to complications in camera design and in design of a suitable CRT. Such a panoramagram television system would require many, perhaps dozens of channels, to overcome viewer eye placement restrictions.
A more practical approach is to use two picture channels for transmission of a conventional stereopair to be viewed with individual selection devices. Such art has been well travelled but the method of display remains a serious challenge to the ingenuity of the inventor. For economic reasons it would be good to depart as little as possible from the CRT designs commercially available, but many workers have advocated various special types of tubes or viewing systems. Designers are faced with the fact that the rear screen projected image of the familiar CRT picture tube uses electrons, not light, and electrons cannot be encoded with polarization information. Therefore, attempts must be made to polarize or encode the image after it has been formed by the CRT. U.S. Pat. No. 3,858,001 (Bonne) uses an electro-optical device in front of the surface of the screen to alter the axis of polarization of light in synchronization with incoming alternate left and right image fields. U.S. Pat. No. 4,122,484 (Tan) suggests using features of rear and front screen displays employing a CRT combined with a front projection scheme.
These two ideas illustrate prior art concerns and approaches. In one case (Bonne), left and right images are displayed alternately, with appropriate polarization characteristics added at the surface of the screen in synchronization with alternate image fields. In another case (Tan), a CRT has its image encoded with a sheet polarizer which is superimposed with a video projected image of light orthogonally polarized with respect to the CRT image.
Other designers have used special tube designs to steer the image elements to the appropriate position on the face of the CRT in order to place those elements in proper juxtaposition with sheet polarizers. U.S. Pat. No. 2,783,406 (Vanderhooft) shows such a tube using internal vertical masking to steer the image, similar to designs used for color tubes. (This particular embodiment is shown applied to an interdigitized parallax stereogram but would apply as well to alignment of vertical image elements with strips of sheet polarizers.)
Another approach, which has been revived by the introduction of electro-optical shuttering devices, is the eclipse method of image selection. In the eclipse method right and left shutters alternately occlude the unwanted image and transmit the wanted image.
U.S. Pat. No. 1,435,520 (Hammond) shows such a mechanical shuttering system for motion picture work and others, like U.S. Pat. No. 2,810,318 (Dockhorn) similar mechano-optical systems for television, but mechano-optical occluding devices are bulky and cumbersome. One major disadvantage of all occlusion systems is that left and right stereoscopic moving images must be photographed or televised and presented essentially simultaneously. Jones and Schurclif, writing in the Journal of the S.M.P.T.E. (1954, Vol. 62 p. 134) point out that shutter phase for motion picture projection must be to within 100 degrees (or 0.01 second) of the 360 degrees of the intermittent cycle. My investigations bear this out, and the shutter synchronization for photography must be to even closer tolerances, of about 10 degrees (or 0.0008 seconds) for fast moving subject.
With the occulsion system proposed by all prior television art, spurious temporal effects are to be expected since left and right images are taken and presented completely out of phase, or 0.02 seconds apart. Such spurious effects arise because objects will move some finite distance in the time interval between right and left shutter or scanning operation, and this will produce unwanted motion of the right with respect to the left image field. This is interpreted as stereoscopic information since it is turned into parallax information.
In the modern occluding systems electro-optical shutters replace mechano-optical shutters. This is typified by U.S. Pat. No. 3,821,466 (Roese), where alternate fields are used to encode right and left image pairs. This use of alternate fields has a long tradition in the prior art. For example, U.S. Pat. No. 2,578,298 (Goldsmith) proposes such a system. In addition to the aforementioned spurious temporal parallax difficulties, which is described by many viewers as "a kind of unsharpness," such alternate interlace schemes reduce the number of images reaching each eye by half, and create an even more serious problem; image flicker. A flickering image is never desirable in any imaging system, but alternating right and left images carried by odd and even fields will reduce the total number of fields perceived by each eye to well below the critical fusion frequency (CFF). This CFF cannot be specified with a single number since it varies with image brightness, portion of the retina receiving the image, and other factors. However, the number of images for television and motion picture applications is generally set at from 48 to 72 per second for flickerless display.
In addition, existing electro-optical shuttering glasses have low transmittance leading to exceedingly dim images which necessitate viewing the screen in a darkened room.
Prior art such as U.S. Pat. Nos. 3,737,567 (Kratomi) and 3,821,466 (Roese) commonly specified conventional twisted nematic liquid crystal devices but their defect, namely slow decay time, produced cross talk of left through right image and vice versa. Replacing liquid crystal with faster responding ferroelectric lead zirconate-titante ceramic material (PLZT) solved the ghosting problem, but left the display with a rather dim image, since the PLZT material, like the conventional liquid crystal material has a relatively low transmittance.
Field storage figures prominently in my disclosure, therefore it will be worthwhile to review what suggestions have been made vis-a-vis three-dimensional television. In the DOTS (digital optical technology system) as reported in IEEE Spectrum (N. Mokhoff, February 1981) buffers are used to store fields and to compare two stored fields in order to extract a color difference signal to produce a synthetic stereoscopic effect. Although a conventional television may receive such a transmission, complimentary colored glasses must be used as is the case in anaglyph systems. This prevents display of true color images and leads to eyestrain in many viewers.
U.S. Pat. No. 4,214,267 (Roese et al.) teaches that field stores may be used to hold stereofluoroscopic images. The purpose is to hold still images so that they may be displayed conveniently, and a magnetic disc system is advocated.
Zammit and Swegle (Applied Optics, Vol. 18, No. 5, Mar. 1, 1979) discuss a helmet stereoscope display unit worn by the viewer, with CRT's built into it, one for each eye. They advocate using the right-left scheme of alternate odd-even fields well known in the art in order to preserve present bandwidth requirements. They are aware that reducing the number of fields to each eye by half will produce flicker, and they suggest an electronic analogue delay line might be used to show each frame twice on each CRT. Following up this work, Charman (Applied Optics, Vol. 20, No. 1, Jan. 1, 1981) suggests the use of occluding spectacles and a single screen, but he feels that an electronic analog delay line is not necessary since his experiments have suggested that the level of flicker was acceptable because binocular stimulation with images 180.degree. out of phase leads to a reduction in the CFF. Charman was using the 50 Hz British system, in which each eye would have received 25 pictures per second. This observation runs counter to mine in which 30 television fields per second seemed to me to be entirely unacceptable in such a display system because resultant flicker is terribly annoying.